Hundreds of organic reactions fall into a handful of types. Bonds break in one of two ways, attacking species come in two kinds, and curly arrows show exactly where each pair of electrons goes.
In homolytic fission a covalent bond breaks with one electron going to each atom, giving two free radicals (species with an unpaired electron). In heterolytic fission both electrons go to one atom, giving ions. A nucleophile is an electron-pair donor that attacks a δ+ carbon (OH−, CN−, NH3, H2O); an electrophile is an electron-pair acceptor attracted to regions of high electron density such as a C=C (H+, Brδ+, NO2+). A curly arrow shows the movement of a pair of electrons, starting from a lone pair or bond.
Classify each reaction by what happens overall before thinking about mechanism.
| Type | What happens | Example |
|---|---|---|
| addition | two molecules join to give one | C2H4 + Br2 → C2H4Br2 |
| substitution | an atom or group is replaced | CH4 + Cl2 → CH3Cl + HCl |
| elimination | a small molecule is removed, forming C=C | C2H5Br + OH− → C2H4 + H2O + Br− |
| hydrolysis | bonds broken by reaction with water | ester + water → acid + alcohol |
| condensation | two molecules join, losing water | acid + alcohol → ester + water |
| oxidation / reduction | gain of O or loss of H / the reverse, shown by [O] and [H] | ethanol + [O] → ethanal + H2O |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
A nucleophile is:
Homolytic fission of a covalent bond produces:
The reaction C2H4 + H2O → C2H5OH is:
What does a curly arrow represent?
Practise drawing curly arrows in the simulator. Each arrow must start at a pair of electrons and end where that pair forms a new bond or becomes a lone pair.
| Mechanism | Attacking species | Example |
|---|---|---|
| free-radical substitution | Cl• radical (homolytic fission in UV) | methane + chlorine |
| electrophilic addition | electrophile, e.g. Brδ+ | ethene + bromine |
| nucleophilic substitution | nucleophile, e.g. OH− | bromoethane + aqueous NaOH |
| nucleophilic addition | nucleophile, e.g. CN− | ethanal + HCN |
A nucleophile donates an electron pair; an electrophile accepts one. Curly arrows start from a lone pair or a bond, never from an atom symbol or a charge sign. Homolytic fission gives radicals, not ions.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which species is an electrophile?
Which type of reaction and mechanism occurs between bromoethane and aqueous sodium hydroxide?
Where must the first curly arrow start when OH− attacks bromoethane?
Which is a hydrolysis reaction?
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